1. Field of the Invention
This invention relates to an electron emitting cathode for emitting photoelectrons or secondary electrons, which may suitably be used in a photomultiplier tube or other devices.
2. Related Background Art
A photocathode has a characteristic that its electron emission efficiency differs for the polarization state of the incident light even if the intensity of the incident light is the same. This is because photoelectric emission depends on an angle between the photocathode and the plane of polarization of light, hereinafter referred to as a characteristic of polarization of light. In the measurement of light, the intensity of incident light is usually one feature that is measured so that the characteristic of the photocathode is disadvantaged. In order to avoid this disadvantage, the photocathode is formed of an aggregate of small particles, and a surface of the photocathode is made to form a variety of angles against a plane of polarization of incident light. Then, the characteristic of polarization of light is practically eliminated for the entire photocathode. More particularly, the layer of solid structure formed by a large number of microscopic particles similar to a cavernous body, which has hollow inside, is formed on an electrode. A photocathode comprising one or plural alkali metal(s) and an antimony metal is appreciated as an electron emitting cathode to be used in a phototube or photomultiplier tube. Hence the photocathode may also be operated as a secondary electron emitting cathode, this is used as a dynode of a photomultiplier tube or secondary-electron multiplier tube. The difference between photoelectric emission phenomenon and secondary electron emission phenomenon is that a cause of electric emission is respectively an incident light and an electron input, although basically most characteristics of these phenomena are in common.
More particularly, for example, for a 1 (1/8) inch photomultiplier tube, an Sb layer having a thickness of about 2000 angstroms is deposited on a surface of a Ni base substrate as a photocathode, and it is activated with an alkali metal. Here, in case of a bialkali photocathode, its luminous sensitivity is 30-70 .mu.A/lm and in case of a multialkali photocathode, 50-100 .mu.A/lm is obtained. An Al base substrate has been developed to improve the sensitivity, and its luminous sensitivity is increased to 200 .mu.A/lm. The following literature is cited as a reference relating to the Al base substrate.
"Photoemissive Materials" by A. H. SOMMER (JOHN WILEY & SONS. INC. 1968)
Further, as a photocathode having another structure, a photocathode formed by depositing an Al.sub.2 O.sub.3 layer and an Sb layer on a Ni base substrate is disclosed in U.S. Pat. No. 4,160,185 by RCA INC. Here, the Al.sub.2 O.sub.3 layer has a function of preventing the Ni base substrate and Sb from alloying. Further, a structure made by forming a porous Sb layer on an Al base substrate through a solid-Sb layer is disclosed in GB Patent No. 1,503,875 by RCA INC. Moreover, in order to improve the sensitivity to long wavelengths, a structure comprising a photocathode placed on an Al layer with a pattern of islands, stripes or slits is disclosed in Japanese Laid-Open Patent Application No. 22858/1974.